B747-400
Boeing state that if fuel is the ONLY consideration, then fly at LRC which is M.859 at OPTIMUM. We fly ECON CI = 60 which is about M.845 at OPTIMUM.
On a LHR - SFO - LHR the difference is about 15 mins a day, 91.25 hours per year per aircraft. That 91.25 will give at least 4 more LHR - SFO - LHR trips with all that revenue in the kitty before the next heavy check (assuming the check is on flight hours). How many crews change the step size from ICAO to 2000' and fly at optimum or just above? How many crews adjust the Mach number to LRC if held down to a lower flight level? How many crews consider what effect a CI of 60 has on the climb and descent? How does the company calculate the CI?...getting the information from engineering is like getting blood out of a stone. The fact is that every aircraft is designed to have a sweet spot and the B747-400 does not like getting slow. The difference in fuel burn between M.86 and M.85 is minimal but remember...
TIME IS MONEY.